Is it permissible to slaughter a cow at the price of four sheep in fulfillment of a vow to slaughter two sheep, with the desire to add two more sheep, and is it permissible to eat from it or must it all be given as charity? And is it permissible to buy a male goat instead of sheep?

A vow is not legally mandated, but whoever vows obedience to Allah is obliged to fulfill it, such as sacrificing a sheep upon being blessed with a male child. The vowed sheep must be fulfilled as it is, and it is not sufficient to substitute it with a cow or a male goat. It is permissible to eat from the vowed sheep unless the intention was to give it solely to the poor; if that was the intention, then it is not permissible to eat from it. As for the second sheep, it is a supererogatory act, and the remaining two sheep are for the 'Aqiqah.